In semiconductor technology, image sensors are used for sensing light emitted towards them to form an image. The image sensor includes a dummy wafer and a sensing wafer bonded on the circuit wafer. The sensing wafer includes pixel units for converting various types of photo energy of light into electrical signals, and peripheral circuits of the pixel units. For example, the peripheral circuits include source followers to amplify the electrical signals.
In general, the image sensor may be a complementary metal-oxide-semiconductor (CMOS) image sensor (CIS), an active-pixel sensor (APS), a passive-pixel sensor and a charged-coupled device (CCD) sensor. The above image sensor is widely used in various applications such as digital camera or mobile phone camera devices.